The manufacturers of photovoltaic devices (PV), commonly referred to as solar cells, have historically been made from high purity single crystal, lattice matched semiconductor alloys where, due to the absence of dislocations, very few traps were encountered by free electrons generated in the photo voltaic process. The only traps existed at the surface between the device and air. These traps were generated by the surface configuration of the dangling bonds. These occurred as the three dimensional crystallographic nature of the atoms terminated at the air/crystal interface.
To reduce or eliminate these traps, manufacturers used additional thin layers on the front and/or back surfaces. These additional layers would have excellent passivating characteristics to the device structure removing the traps from the device. The passivating layers reduce the carrier recombination at silicon surfaces and therefore result in higher open-circuit voltage, which becomes increasingly important for high-performing solar cells. If applied on the front surface of the solar cells, the passivating layers can act as anti-reflective coatings (ARC).
On the other hand, when the coating layers are applied at the backside of solar cells, the optical parameters of this additional layer could be designed in such a way to improve the internal reflectivity in the device to enhance the light absorption by the device, and as a consequence, increase the efficiency of the device. Traditional surface passivation materials include SiOx, SiNx and etc which are usually prepared using vacuum techniques, such as, thermal oxidation, PECVD, or sputtering. Recently, AlOx draws more attention both academically and in industry. It has been demonstrated that amorphous AlOx films prepared by Atomic Layer Deposition (ALD) or Plasma Enhanced Chemical Vapor Deposition (PECVD) yield an excellent level of surface passivation on c-Si. However, due to the complexity of the PECVD and other processes, and the relatively high cost of the manufacturing tools and consumable chemicals and gases (eg. Trimethylaluminium or TMA), it is of great importance to develop alternative solutions to form dielectric coatings for silicon wafer solar cells. Liquid phase deposition is such a competitive approach which is of comparable passivation quality, cost-effective, and environmental friendly. With that, coatings prepared under atmospheric conditions are possible by eliminating those complex and expensive manufacturing tools.
The present invention aims at providing methods of modifying silicon solar substrates by liquid phase deposition while avoiding the use of hazardous chemicals and conditions.